Preparation of salt mixtures



Patented Aug. 24, 1937 UNITED STATES PATENT OFFICE PREPARATION OF SALTMIXTURES No Drawing. Original application July 29, 1933,

Serial No. 682,883. Divided and this application August 29, 1935,

Claims.

This invention relates to amethod for the purification of salt mixturesand more specifically to the preparation of a mixture of salt suitablefor the production of light metals byiused salt elec- 5 trolysis.

This application is a divisional of my copendirgg application, SerialNo. 682,883, filed July 29, 1 33.

In the production of light metals by the electrolysis of fused salt itis common practice to use a mixture of salts rather than a pure fusedsalt, chiefly in order toobtain a low-melting electrolyte. For example,in the production of sodium by the electrolysis of sodium chloride oneor more salts of other metals are added. Thus a commonly usedelectrolyte for the manufacture of 7 sodium by a fused salt electrolysisconsists of a mixture of sodium chloride and calcium chloride.

The calcium chloride used for this purpose must be carefully fused toremove all traces of water before its introduction into the electrolyticcell.

. When a fused mixture of sodium chloride and calcium chloride iselectrolyzed, the sodium produced contains a small amount of calcium.The crude sodium metal, collected from a plurality of electrolyticcells, contains in addition to calcium, oxides and chlorides of thesemetals. Substantially pure sodium is obtained by filtering the moltencrude metal at a temperature at which the calcium is substantiallyinsoluble. The filter residue consists of the oxides and salts, togetherwith large quantities of calcium and sodium.

A proposed process for recovering sodium from a mixture of sodium,calcium and their oxides comprises reactingthe mixture with a fused bathcontaining such quantities of sodium chloride that the calcium reactswith the sodium chloride to set free sodium and form calcium chloride.The liberated sodium is removed from the melt by a gravity method, whilethe calcium oxide remains in the melt in solution and in suspension.Thus the by-product of this process of recovery comprises a mixture ofthe excess sodium chloride, calcium chloride and calcium oxide, sincepractically all of the sodium oxide introduced reacts with the calciumchloride to form sodium chloride and calcium oxide. A certain amount ofthe calcium oxide, depending upon the concentration of calcium chloridein the mixture, is in solution; the remainder is a finely dividedprecipitate which will settle out if the molten bath is allowed to standfor a sufficient time without agitation. The bulk of the precipitatedoxide may be removed by mechanical means from the melt after settling.The resulting melt, after the ad- Serial No. 38,437

dition of more sodium chloride if necessary, may then be used aselectrolyte for the fused salt electrolytic production of sodium.However it is difficult to remove all of the precipitated oxide andfurthermore a considerable quantity of the oxide remains dissolved inthe melt; and the presence of the oxide is generally a disadvantage inthe electrolysis process. The oxide, if introduced into the electrolyticcell, causes a liberation of oxygen, thus contaminating the by-productchlorine with oxygen, and furthermore the oxygen set free tends tooxidize graphite or carbon electrodes when these are used in the fusedsalt electrolysis process. For this reason the salt-oxide mixture has noutility and must be wasted unless the oxide can be removed. Hence, Ihave sought economical means for removing oxides from such salt-oxidemixtures in order to prepare oxidefree salt mixtures suitable for thefused salt electrolytic production of sodium.

One object of this invention is to provide a method for the removal ofcalcium oxide from mixtures of sodium chloride and calcium chloride. Afurther object is to prepare oxide-free, anhydrous salt mixturessuitable for the fused salt electrolytic production of sodium. Otherobjects will be apparent from the following description of my invention.

In one method of carrying out my invention, a mixture of calciumchloride and alkali metal chloride, containing calcium oxide partly insuspension and partly in solution, is subjected to the action of directelectrical current in a suitable fused salt electrolysis cell, forexample, a cell which is suitable for the production of the alkalimetal. I prefer to use the type of electrolytic cell described in DownsU. S. Pat. 1,501,756. The mixture, which may contain up to around 20% ofcalciumoxide, is placed in the cell, fused therein by the action of theelectric current or by other suitable means and the cell is thenoperated in the usual manner. Advantageously, a cell may be selectedwhich is already operating to produce the alkali metal and part of theelectrolyte may be removed and replaced with the oxide-containing saltmixture. As the electrolysis proceeds, alkali metal is produced at thecathode and part of the chlorine evolved at the anode reacts with theoxide to produce calcium chloride and liberate oxygen. As known to theart, if the calcium chloride content becomes too high, relatively largeamounts of calcium will also be produced in the cell. I prefer tomaintain the calcium chloride content sufficiently low so that theminimum amount of calcium is produced.

When an analysis of the salt mixture in the cell shows that the calciumoxide has disappeared or that its concentration has been reduced to thedesired extent, a portion of the electrolyte is re- 5 moved and replacedby a fresh quantity of oxidecontaining salt mixture. The portion ofelectrolyte removed may be transferred to another electrolytic cellwhere it may be electrolyzed to product alkali metal and oxygen-freechlorine, with the addition of more alkali metal chloride if necessary.

My invention is further illustrated by the following example:

Example A Downs-type fused salt electrolytic cell was operated in thenormal manner to produce sodium, using a fused mixture of calciumchloride and sodium chloride as electrolyte. While the 0 cell wascontinuously maintained in operation,

about 2% by weight of the electrolyte was removed from the cell and inits place was introduced about 1.5% by weight of the original.electrolyte, of a salt mixture consisting of about 18% by weight ofcalcium oxide, 50% by weight of calcium chloride and 32% by weight ofsodium chloride, together with sufiicient additional sodium chloride tobring the molten electrolyte up to its original level. During thesubsequent operation of the cell, the electrolyte was maintained at anapproximately constant level by the addition of sodium chloride asrequired.

At the end of one hour and hourly thereafter, for a period of ten days,the above described 35 removal of electrolyte and replacement with thecalcium oxide-salt-mixture was carried out. The data tabulated belowgives the results obtained from two typical runs and the average of theresults obtained over the ten-day period:

Mixture added to cell :gff Sample CaO CBC]: NaGl CaO CaCl; NaCl PercentPercent Percent Percent Percent Percent A 17.9 47.8 34. 3 0.09 58. 2 41.7 B 17.4 50.0 32. 6 0. 14 69. 5 41. 2 Average for 10 days 18 32 0. l 5940. 9

The substantially oxide-free salt thus obtained was moulded into bricksand was fed as required together with suitable amounts of sodiumchloride, to other electrolytic cells to produce sodium. During theten-day period, sufiicient of the substantially oxide-free salt mixturewas produced to supply the entire calcium chloride requirements ofthirty cells of the same size and capacity as the cell used to producethe mixture, over the same period of time.

During the 10-day period the cell used to produce the oxide-free saltmixture produced sodium at the rate of about '70 grams per kilowatt hourof electric current used. The graphite anode in .this cell was corrodedat the average rate of about 0.01 inch per day as compared with a cellutilizing substantially oxide-free electrolyte in which the graphiteanode is corroded at an average rate of around 0.001 inch per day.

In its broader aspects my invention comprises the preparation of a saltmixture suitable for the fused salt electrolytic production of alkalimetals or alkaline earth metals which is accomplished by electrolyzing amixture of alkali metal chloride and/or alkaline earth metal chloridecontaining one or more oxides of the metal or metals, the

chlorides of which it is desired to produce in the mixture beingprepared. Thus, in order to prepare a mixture of calcium and sodiumchlorides to be used in the fused salt electrolytic production of sodiumor in order to produce anhydrous calcium chloride to be later mixed withsodium chloride to produce a suitable electrolyte for the production ofsodium, I electrolyze a mixture of calcium oxide and fused calciumchloride and sodium chloride or a mixture of calcium oxide and fusedcalcium chloride, respectively. I prefer to carry out the process with amixture of salts, since such mixtures ordinarily have lower meltingpoints than a pure salt and hence are more easily electrolyzed. Mymethod of preparing a salt mixture is especially advantageous when oneof the components is hygroscopic, while the corresponding oxide issubstantially non-hygroscopic. For example, either lime or calciumhydrate may be added to. a fused salt bath and electrolyzed with littleor no trouble, while on the other hand calcium chloride must besubstantially competely dehydrated before it can be added to a fusedsalt electrolyte. My process enables the anhydrous calcium chloride tobe made efficiently and economically from the oxide, while at the sametime and during the operation of the same process, sodium and/or calciumis produced in material quantities.

In an establishment where a number of fused salt electrolytic cells areoperated, my process operated in one cell will produce sufficient saltmix-- ture to maintain a number of other cells of approximately the samecapacity in operation. Hence, in such an establishment the disadvantagesdue to the formation of oxygen in my process are reduced to a minimum,as far as the total production of chlorine is concerned; that is, only asmall part of the total chlorine produced will contain oxygen. As iswell known in the art, fused salt electrolytic cells have a certain lifeperiod near the end of which their efiiciency decreases chiefly becauseof corrosion of the parts. I prefer to use my process in such cells nearthe end of their life period in order to minimize the loss due to anodecorrosion by the liberation of oxygen in my process.

I am aware that processes have been proposed heretofore whereby metalsare produced by fused salt electrolysis wherein an oxide of the metal tobe produced is utilized as the raw material entering the fused saltbath. My process is not to be confused with these prior methods ofobtaining a metal from its oxide, which processes do not result in thepreparation of oxide-free. anhydrous salt mixtures. Furthermore, thepurpose of my invention is not to produce metals from their oxides, butto produce a substantially oxide-free salt mixture from an oxide and asalt, in order to produce a mixture which may in turn be separatelyelectrolyzed to produce a metal, preferably a metal other than that ofthe oxide used, and an oxygen-free halogen gas.

fflVIy invention is not restricted to the conversion of calcium oxide tocalcium chloride but is also applicable to the conversion of the oxidesof the other alkaline earth metals, 1. e. barium and strontium and thealkali metal oxides to the respective chlorides. In the appended claimsthe term alkaline oxide is used to designate oxides which 'have adistinctly alkaline reaction when hydrolyzed, namely the oxides of thealkali and alkaline earth metals.

I claim:

1. A process for producing an alkali metal comprising electrolyzing afused electrolyte comprising an alkali metal chloride, alkaline earthmetal chkpride and an alkaline earth metal oxide until the electrolyteis substantially free from oxide, 5 removing a. portion of theoxide-free electrolyte,

adding mits place alkaline earth metal oxide together with alkalineearth metal chloride and alkali metal chloride and continuing theelectrolysis. 10 2. A process for producing an alkali metal comprisingelectrolyzing a fused electrolyte comprising an alkali metal chloride,calcium chloride and calcium oxide until the electrolyte issubstantially free from oxide,'removing a portion of the oxide- I5freeelectrolyte, adding in its place calcium oxide together with alkalineearth .-metal chloride and alkali metalchloride' and continuing theelec-' trolysisf 13. A process iorproducing sodium comprising 20electrolyzing a fused electrolyte comprising sodium chloride, calciumchloride and calcium oxideuntil the electrolyte is substantially freefrom oxide, removing a portion 0! the'oxide-free electrolyte, adding inits place calcium oxide to- 25 gether with calcium chloride and sodiumchloride and continuing the electrolysis.

4. A process for producing an alkali metal comprising electrolyzing aiused electrolyte comprising an alkali metal chloride, alkaline earthmetal chloride and an alkaline earth metal oxide until the electrolyteis substantially free from oxide, removing a portion of theoxide-freeelectrolyte, adding in its place alkaline earth metal oxide togetherwith alkaline earth metal chloride and alkali metal chlorideand-continuing the electrolysis and separately fusing and electrolyzingthe said oxide-free electrolyte to produce alkali metal andsubstantially oxygen-tree chlorine.

' 5. A process for producing sodium comprising electrolyzing a fusedelectrolyte comprising sodium chloride, calcium chloride and calciumoxide until the electrolyte is substantially free from oxide, removing aportion of the said oxideiree electrolyte, adding in its place calciumoxide together with calcium chloride and sodium chloride and continuingthe electrolysis and separately fusing and electrolyzing the saidoxideiree electrolyte to produce sodium and substantially oxygen-freechlorine.

ROBERT EDWIN- HU'LSE.

